Current understanding and optimization strategies for efficient lignin-enzyme interaction: A review

Highlights

• The influencing factors of lignin-enzyme interaction are reviewed.

• Lignin modification strategies and enzyme engineering are discussed.

• Mathematical model and simulation of lignin, cellulose and enzyme are summarized.

• Optimization strategies for promoting the enzymatic hydrolysis are outlined.

Abstract

From energy perspective, with abundant polysaccharides (45–85%), the renewable lignocellulosic is recognized as the 2nd generation feedstock for bioethanol and bio-based products production. Enzymatic hydrolysis is a critical pathway to yield fermentable monosaccharides from pretreated substrates of lignocellulose. Nevertheless, the lignin presence in lignocellulosic substrates leads to the low substrate enzymatic digestibility ascribed to the nonproductive adsorption. It has been reported that the water-soluble lignin (low molecular weight, sulfonated/sulfomethylated and graft polymer) enhance the rate of enzymatic digestibility, however, the catalytic mechanism of lignin-enzyme interaction remains elusive. In this review, optimization strategies for enzymatic hydrolysis based on the lignin structural modification, enzyme engineering, and different additives are critically reviewed. Lignin-enzyme interaction mechanism is also discussed (lignin and various cellulases). In addition, the mathematical models and simulation of lignin, cellulose and enzyme aims for promoting an integrated biomass-conversion process for sustainable production of value-added biofuels.

Introduction

The rapid consumption of fossil fuels and their well-documented adverse environmental impacts put an emergency alert to develop clean and renewable energy [1], [2]. To diminish the reliance on fossil fuels, governments across the globe have launched extensive research into an alternative liquid fuel from renewable and inedible resources. Due to its colossal biomass reservoir in the ecosystem, apart from being eco-friendly, renewable and economically viable, the lignocellulosic biomasses (75% polysaccharides) have been regarded as the most suitable raw material for biofuels production [3]. Lignocellulose is mainly composed of tightly bonded polymers, viz. cellulose, hemicelluloses and lignin; however, the content of three polymers varies among the lignocellulose biomass derived from disparate origins. From bioconversion points of view, the fermentable monosaccharide transformed in a specific biochemical pathway occurs due to the hydrolysis of glycosidic bonds in polysaccharides (e.g., celluloses and hemicelluloses). Lignin is the second rich terrestrial polymer composed of the alkyl-aromatic polymer as the basic structural unit, owns a three-dimensional network linked with hemicellulose structural units and is wrapped around cellulose structure. These linkages via the covalent and non-covalent bonds construct a dense matrix to avoid the attack and degradation of structural polysaccharides by microorganisms to maintain the maintenance of plant cell integrity [4].

For biochemical conversion, the pretreatment is a prerequisite to overcome the inherent stubborn structure of lignocellulose and enhance enzyme accessibility to polysaccharides [5]. The other vital operation of the bioconversion process involves high-cost enzymatic hydrolysis. This step requires the high enzyme dose to achieve a high fermentable monosaccharides yield [6]. For enzymatic hydrolysis, lignin acts as a physical barrier and thus decrease cellulose accessibility [7]. In addition, the enzymes inevitably adsorb onto the substrate lignin limiting the efficiency of enzymatic hydrolysis [8]. So far, three types of interactions of lignin adsorbing enzymes, i.e., hydrophobic, electrostatic and hydrogen bonding, have been proposed [9], [10]. Among these interactions, the hydrophobic interaction is highly recognized, meanwhile, the theory of electrostatic interaction is widely reported in the literature of lignin-enzyme complexation [11], [12]. To weaken or eliminate insoluble substrate lignin that nonproductively adsorbs on cellulases, extensive researchers have been carried out to optimize the pretreatment and enzymatic hydrolysis process. Pretreatment reduces the size of lignocellulose biomass particles, unwind the original recalcitrant structure of lignocellulose and reduce lignin to a large extent [13]. Pretreatment process leads to increased accessibility of cellulase to lignocellulose substrates due to the reduction of the interference of substrate lignin on cellulase in subsequent enzymatic hydrolysis [14], [15], [16]. However, it is rare that the lignin is eradicated via pretreatment, therefore, the nonproductive adsorption between residual lignin and cellulase results in the limited enhancement of enzymatic saccharification.

Additives play a crucial role in reducing the nonproductive adsorption of lignin on cellulases. Moreover, no harmful substances are produced by addition with additives. Various additives have disparate mechanisms of reducing nonproductive adsorption to increase the monosaccharides yield. Examples of these different additives include the surface active agents (PEG, urea, Tween, CTAB), non-catalytic protein (BSA, peanut protein), edible food waste (Tea saponin), etc. On the one hand, steric repulsion exists between enzyme and the lignin surface due to hydrophobic interaction, electrostatic interaction and/or hydrogen bonding between bulk lignin and the surfactant [17], [18]. On the other hand, some additives increase the cellulase activity in a reaction mixture [19]. And some mixing of additives with cellulase can form a complex with a higher degree of freedom [20]. Besides, the value-added utilization of lignin can also make it an ideal adjuvant that optimally increases the enzymatic hydrolysis. Modification of functional groups in lignin structure yields water-soluble lignin, which effectively reduces the inhibition of enzymatic hydrolysis [21]. And grafting some polymers on lignin imparts new available properties that change its original negative character on enzymatic hydrolysis [22]. Additionally, enzyme engineering is also an efficient method to improve enzymatic hydrolysis performance by modifying the structure of cellulases [23].

Based on the molecular mechanism of cellulase hydrolysis, this review deeply interprets the influence of lignin on lignocellulose hydrolysis, as well as the interaction between lignin and cellulase, especially the fundamental mechanisms associating the characteristics of lignin-related viz. lignin content, molecular weights, S/G ratio, and characteristic functional groups. In addition, the effects of the trait of cellulase-related, i.e., types, modules, and amino acid residue on lignin adsorption are described. An overall comprehensive understanding of enhancing enzymatic hydrolysis on pretreatment, lignin modification, biological modification and additives are also discussed.